1. Field of the Invention
The present invention concerns a method for calibration of an image-generating x-ray system with a digital x-ray receiver. The invention moreover concerns a method for generation of an x-ray image with an x-ray system calibrated according to such a method as well as an x-ray system operating according to such a method.
2. Description of the Prior Art
The digitized measurement signals (known as raw image data or raw images) generated from the detector elements of a digital x-ray receiver (which detector elements are arranged in a two-dimensional array) must be post-processed and corrected in an image processing device for reproduction of the x-ray image on a reproduction medium that is suitable for use by a radiologist from a diagnostic standpoint. In this post-processing, both the dark current component (offset) and the sensitivity (gain) must be taken into account and corrected for every single detector element.
The sensitivity of the detector elements is thereby normally not constant over the entire relevant dose range (dynamic range), i.e. the range of the dose settings with which the x-ray system can be operated, such that a calibration must be made for a number of dose settings (i.e. a number of values of the current (anode current) flowing to the anode in the x-ray tube given a constant x-ray voltage (constant x-ray spectrum), in order to achieve a linear response of the detector elements of the digital x-ray receiver to the incident x-ray radiation.
By means of such a multi-point calibration, it is ensured that a homogeneously exposed x-ray image (bright image) is always provided by the x-ray receiver at each dose setting in the absence of a subject, except for the quantum and electronic noise.
This calibration, however, is only exactly valid for a constant temperature. If a temperature difference exists between the point in time of the calibration (which normally is implemented after the expiration of a warm-up phase) and the point in time of the actual image acquisition (which, for example, is effected shortly after activation of the x-ray system), an error results for each detector element that is specific to this detector element, since its sensitivity no longer coincides with the sensitivity that it had during the calibration. These differences lead to artifacts in a two-dimensional x-ray image or in a three-dimensional reconstruction. In the case of a two-dimensional image, an intensity gradient results primarily in the direction of an image coordinate. Line artifacts that become noticeable as a radial intensity gradient and ring artifacts occur in a 3D reconstruction.
In principle it would now be possible to take this temperature dependency of the sensitivity of the individual detector elements into account by a calibration at different temperature ratios being done at regular time intervals in the environment of the x-ray receiver, and storing the determined data in a memory.
A number of temperature sensors is arranged in the environment of the x-ray receiver to determine respective existing temperature ratios. A characteristic current reference temperature representative of the current temperature ratios in the x-ray receiver is formed from the temperatures measured at various measurement points. These temperatures normally are not the same over the entire area (surface) of the x-ray receiver, but rather can exhibit a significant difference caused by electrical components arranged in the environment of the x-ray receiver.
If a series of x-ray images were then acquired during a warm-up phase, using the respectively determined current reference temperature could be checked whether a calibration exists for these images.
In practice, however, a multi-point calibration for a number of reference temperatures is nearly impossible because the total duration of the calibration also increases with the increasing number of the measurement points (dose settings) used for the calibration at a reference temperature.
In order to be able to calibrate exactly, a waiting time on the order of 5 to 10 minutes must elapse between the individual dose settings in order to reduce interference effects (such as, for example, persisting luminescence).
The pure measurement time for a ten-point calibration, for example, thus lasts at least one hour. During this time a constant reference temperature of the x-ray receiver cannot be assumed, such that in principle the heating procedure must be reproducibly repeated for every dose setting. Given a heating or cooling phase of approximately four hours that is necessary for this, only one dose setting per day could be accommodated. A ten-point calibration that takes into account the temperature effects would thus take up 10 days. Such a calibration cannot be implemented in practice.